With the growth of electronics technologies, and amidst the progressively increasing power output of semiconductors, aluminum nitride substrates with excellent insulation properties used in circuit boards for semiconductor mounting are being used in various fields, e.g., as substrate materials used in the drive control of trains or electric automobiles or in the control of industrial robots. Among them, highly reliable SiC chips are promising materials for replacing current Si chips in order to develop the next generation of semiconductors with the characteristics of reduced switching loss or energy loss which greatly affects product reliability and expanded control operation temperature. The operable temperature of a SiC chip is around 400° C., which is higher than the conventional 150° C., so the aluminum nitride substrate used as an insulating material of a circuit board for semiconductor mounting also needs to exhibit excellent insulation properties at such high temperatures.
Conventionally, the aluminum nitride sintered bodies used as the above aluminum nitride substrates have generally been produced by the following method. Specifically, additives such as sintering aids, binders, plasticizers, dispersion media or mold release agents are mixed with an aluminum nitride powder. The mixture is formed into a sheet by extrusion molding etc., and processed (formed/pressed) into a desired shape and dimensions by a pressing machine. Next, the compact is heated to 350 to 700° C. in air or in a non-oxidizing atmosphere such as nitrogen to remove the binder (debindering), and then held for 0.5 to 10 hours at 1800 to 1900° C. in a non-oxidizing atmosphere such as nitrogen (sintering) to produce a sintered body.
However, while the breakdown voltage of an aluminum nitride substrate produced by this method is approximately 30 to 40 kV/mm at room temperature, thus exhibiting high insulation properties, there was the problem of it decreasing to approximately 10 kV/mm at high temperatures such as 400° C.
Various proposals have been heretofore made in order to increase the insulation properties of an aluminum nitride sintered body. For, example, a method for increasing unpaired electron concentration by forming a solid solution of titanium in aluminum nitride crystal grains (Patent Document 1) and a method for controlling the average diameter of aluminum nitride crystal grains or grain boundary pores and the ratio between grain boundary pores and intragranular pores (Patent Document 2) have been proposed. However, nothing until now has been able to ensure insulation properties at a high temperature.